Antenna system



l March 1947. G, H, BRWN 2,417,290

ANTENNA SYSTEM f Filed Feb. 25, 19g?,

/ Gttorneg Patented Mar. 1l, 1947 ANTENNA SYSTEM George H'. Brown,Princeton, N. J., assignor to Radio Corporation of America, acorporation cf Delaware Application February 23, 19.43, Serial No..416.820

This invention rela-tes to antennas, and more 'particularly tostructuresV for radiating energy with high efficiency over a broad bandci ultra high frequencies.

It is the principal object of this invention to provide an improveddipole antenna structure having a broad resonance characteristic.

Another object is to provide an improved dipole antenna which ismechanically strong, of light" weight, and simple in construction.

A further object is to provide an improved dipole antenna which may becoupled to a standard concentric transmission line without requiringtransformers or complex networks.

These and other objects will become apparent 1 to those skilled in theart upon consideration of the following description with reference tothe Fig. 1.

Referring to Fig. l, a pair of parallel tubular supportingmembers I and3 are fastened by welding, for example, to a base plate 5. Tubularradiator elements 1 and 9 are similarly secured to the supports I and 3at the upper ends, as shown more clearly in Fig. 2. The members I, 3

and elements 1, 5 may be made of thin-walled y steel tubing or the like.Metal bushings Il and I3 are inserted at the upper ends of the supportsI and 3. Coaxial lines l5 and I1 extend through the supports I .and 3respectively and the bushings II and I3. The outer conductors of thelines are connected to the bushings. Insulatng caps I9 and 2l areprovided for the bushings H and I3, and conductors 23 and 25respectively are supported by and extend through the Caps.. The innerConductors of the lines I5 and l1 are connected to the conductors 23 and25. The conductors 23, 25 are connected together through a snorting bar21.

The dimensions of the various parts in terms of the wavelength, A, atthe center of the band over which the system is tocperate, are indicatedin Fig. 1. The supports I and 3 and the radiator elements 1 and l'i areeach substantially one quarter wavelength long. The line l5, from theterminal 23 to the junction point 29, is electrically 5 Claims. (Cl.25u-33) one half wavelength longer than the line I1. The

physical difference in length between the lines I5 and I1 will besomewhat less if the velocity of propagation of energy through the linesis less than that in space. A line 3l is connected to the junction 29and leads to a source of energy or radio device, not shown. The baseplate 5 may be grounded, as by connection to a. metallic sup'- portingframework or to a reiiector, and the outer conductors o f the lines I5,I1 and 3l are also grounded at any convenient points.

The operation of the above described' structure is substantially asfollows:

Radio frequency voltage is applied between the inner and outerconductors of the lines I5 and I1 through the line 3l. At the instantthe inner conductor of the line I1 is positive at the point 33 at thebase 5, the inner conductor of the line l5 is negative at the point 35,`because the length of the line I5 between the points 29 and 35 is 180electrical degrees longer than that of the lineI1 between the points 29and 33. Since the ends of the inner conductors of the lines I5 and I1are connected together by the shorting bar 21, cur-.- rent flows upthrough the inner conductor of the line I1, across the bar 21, and downthrough the inner conductor of the line I5. The above conditions arereversed with each R.F. cycle. The currents flowing in the innerconductors of the lines I5v and I1 induce opposite currents in therespective outer conductors. Thus the instan! taneous flowof current onthe outer conductor ci the line l5 is upward when that on the line I1 isdownward. The supports I and 3 constitute a parallel line, shortcircuited by the base 5, and thus present a high impedance between thepoints 31 and 3S, at the ends ofthe outer conductors of the lines I5 andI1. The voltage between these points is applied to the dipole 1, 9.

The .dipole is equivalent to a series resonant circuit comprisingcapacitance, inductance, and resistance. Near the resonant frequency,the et, fective resistance between the inner ends of the radiatorelements is 36 ohms. The parallel line formed by the supports I and 3 isequivalent to a parallel resonant circuit. With the dimensionsindicated, the resonant frequency of the dipole is slightly lower thanthat of the parallel line, so that at a frequency at which the dipoleexhibits, for example, 20 ohms inductive reactance, the line hassuiiicient capacitive reactance to resonate the dipole. At thisfrequency the line and the dipole together act like a parallel resonantcircuit connected between the points 31 and 39 with the re- 3 sistancein the inductive arm, and present a purely resistive load of R2 X5l l cZ= i which in this case is about 50 ohms, and is substantially equal tothe surge impedance of the coaxial lines l and Il. At other frequencies,the reactance of the dipole is still opposite'in sign to that of theparallel line, since as the frequency is increased the series resonantcircuit becomesV more inductive and the parallel resonant circuitVbecomes more capacitive, and vice versa. Thus' the combined impedance ofthe radiator and the support is a relatively constant resistance over a.

wide frequency band, Vproviding uniformly high efficiency of radiation.Y i

If the lines I5 and Il match the dipole impedance, as described above,the impedance looking toward the antenna from the junction point -29-is' one-halfA the line impedance, or in the example given above, 25ohms. To prevent re- Yilection at this point, the impedance looking intothe line 3| Vmust also be 25 ohms. If a single dipole is to be used, a25 ohm line may be employed, or impedance matching means may beAprovided on the line 3 I.

Referring to Fig. 4, a pair of dipoles like that shown in Fig. 1 may beconnected together through quarter wave lines lll and 43 to a main line45. With this arrangement all of the lines may have the samecharacteristic impedance and no further impedance matching means isrequired. If the impedance looking towards the antennas from thejunction points 29 and 29 is 25 ohms, the impedance looking into each ofthe quarter wave lines 4l and 43 from this junction with the line 45 is:

2 2 n Z in= zal' or =l00 ohms The impedance of the two lines in parallelis then 50 ohms, which matches the line 45.

Referring to Fig. 3, the curve lll represents the standing wave ratio ina line feeding the antenna system of Fig. 4, as a function of frequency.This ratio is a measure of the impedance match between the antenna andthe line. Thus, if the ratio is 1, the impedances are perfectly matched,and substantially all of the energy fed into the line is applied to theantenna. Fig. 3 shows that this ratio remains above 'l0 precent over therange 'of 156 megacycles to 188 megacycles, for an i4 Y tice, theradiator elements are connected to the outer line conductors and theinner line conductors are connected to each other at the center of thedipole. nected together one quarter wavelength from the ends of thelines, and function asa short cirouited quarter wave parallel lineshunted across the dipole. Currents flowing through the inner conductorsinduce currents in the outer conductors, providing coupling to thedipole. The quarter wave line provides loading Vfor the dipole tobroaden the resonance and maintain substantially constant and resistiveimpedance over a Y wide frequency band.

` I claim as my invention:

1. A dipole antenna comprising a pair of parallel tubular supportingmembers, radiator eley ments secured in end-to-end relationship at theends of said supporting members, and coaxial transmission linesextending through said supports with their outer conductors connected tosaid supports and their inner conductors connected together near theadjacent ends of said. radiator elements. k

2. A dipole antenna comprising two radiator elements disposed colinearlyin end-to-end relationship, two concentric lines with the ends of theirouter conductors connected respectively to said radiator elements andthe ends of their inner conductors connected together, and a connectionbetween the outer conductors of said lines at a point substantially onequarter wavelength from said ends.

3. A dipole antenna comprising a pair of parallel tubular supportingmembers substantially one quarter wavelength long, radiator elementseach substantially one quarter wavelength long connected in end-to-endrelationship at the upper ends of said supporting members, a plate ofvconductive material connected at the lower ends of said supportingmembers, and coaxial transmission lines extending through said supportswith their outer conductors connected to said supports and the ends oftheir inner conductors connected togetherwsaidV transmission lines dif`fering in length by substantially electrical degrees.

4. An antenna array comprising at least one pair of dipoles, each ofsaid dipoles including tubular supporting members, coaxial transmissionlines extending through said supporting members with ends of their outerconductors connected to said supports and the ends of their innerconductors connected together, said lines differing in length bysubstantially one half wavelength and being connected together to abranch line of substantially one quarter wavelength, the two branchlines for each pair of dipoles being connected together to a common mainline. n

5. The invention as set forth in claim 4 wherein said tubular supportingmembers are substantially one quarter wavelength long and are connectedtogether at their ends remote from the radiator elements of saiddipoles. Y

GEORGE H. BROWN.

The outer conductors are con-Y

